The Louisville Twin Study (LTS) was established in 1959, and continuously funded for forty years; data collection was terminated in 2003. The LTS includes data on over 1500 twins born in the Louisville area. Data were collected on physical and medical parameters, health, families, school performance and cognitive ability at 16 different measurement occasions between three months and fifteen years of age. Repeated measurements of cognitive ability will be used to answer important questions about the causes of differences in ability through development. Data will be fit to an integrative model that will provide explanations of some of the most important observations regarding intellectual development during childhood: the importance of genetic factors increases throughout childhood, the role of genetics is diminished among children raised in impoverished environments, and the temporal stability of individual twins and siblings relative their family members increases with age. The hypothesis is that all three phenomena occur as a result of a process of ongoing matching of children to environments, through which children with the greatest observed cognitive abilities are exposed to the most enriching environments. In addition, funds are requested to support the ongoing recovery and archiving of the data from the LTS, so they can eventually be made available to the scientific community at large.